For many data processing applications, there is a need to transmit information between a data source and a display or between two memory-containing devices. The term "information" as utilized herein includes commands or instructions as well as data. One example of such data processing applications is that described in U.S. patent application Ser. No. 07/798,295, filed Jul. 27, 1987 for "Miniature Video Display System" by Allen Becker, now U.S. Pat. No. 4,934,773 and/or in U.S. patent application Ser. No. 07/200,645 filed May 31, 1988 for "Low Vibration Resonant Scanning Unit for Miniature Optical Display Apparatus" by Benjamin A. Wells now U.S. Pat. No. 4,902,083, both of which patent applications are hereby incorporated by reference herein.
These latter patent applications disclose miniature displays which need to be connected to a host computer or other source of data that generates and formats the displayed information. Conventionally, the information to be displayed is periodically sent from the host computer to the display whether or not the displayed information changes so that the display is periodically updated or "refreshed". The information is typically sent from the host to the display as a "bit map", in which each bit corresponds to a picture element in the display. Since a high-resolution display contains many such picture elements, and the refresh interval is generally short (often on the order of five milliseconds) a high peak data rate results (illustratively on the order of 50 MHz).
Where it is desired to provide data from a host computer to a refreshed display at a high data rate, several problems arise. First, the cabling for the data transfer is expensive. If a single cable is used to send the data via serial transmission, then a coaxial cable is required, as well as high-speed drivers and receivers. Alternatively, if the data is transmitted by parallel transmission, then multiple cables are required (although each cable can transmit at a lower data rate and can be driven by lower speed drivers and receivers) and the overall diameter of the cable bundle increases. Also, each separate cable in the bundle should either be a coaxial cable or else the entire bundle should be provided with an outer shield to keep RFI emissions to acceptable levels. The shielding further increases the cable bundle size and a larger cable bundle may be undesirable if the display itself is physically small, such as a heads-up or goggle-mounted display.
Another problem resulting from high data rate operation is the power dissipated when the cable is driven and the capacitance of the cable is charged and discharged every time the signal level rises and falls. For example, an eight foot long cable, with capacitance of 36 picofarads per foot, will dissipate over 1/3 watt when transmitting a 50 MHz TTL square wave. The dissipated power may, in some circumstances, be more power than that used by the display to which the cable is connected.
Accordingly, it is the general purpose of the present invention to provide apparatus and a method capable of providing controlled transfer of information from one device to another.
It is another object of the present invention to provide apparatus and a method for data communication that reduces the power wasted driving the communication cable and eliminates the need for multiple data cables or for a single very high speed link.
Yet other objects of the present invention are to provide apparatus and a method for data communication which allows updating the receiving device's bit map starting at an arbitrary byte boundary with an arbitrary number of bytes being updated, which allows passing commands or instructions such as status and control information to the second device using the same signal path used for transmission of data, which allows any data rate from zero to a maximum value and which allows the data transfer rate to change at any time without substantial risk of loss of the transferred information, which is independent of the resolution of the display or other second device, and which can transmit color and intensity information in digitally coded form.
Further objects of the present invention are to provide apparatus and method for information communication which is capable of a wide range of data transfer rates, which does not require a substantial number of data lines for such data transfer, which has reduced cable power consumption requirements, which is capable of transmitting commands or instructions as well as data, which is capable of transmitting data to specific address(es) or location(s) in the device receiving the data, which is capable of providing data transfer for all or a portion of the storage capacity of the device receiving the transferred data, which has reduced cabling cost requirements, which prevents loss of information if either device is not ready to transmit or receive such information, which is capable of repeatedly returning to a presettable starting address for transfer of data, and which does not require high data transfer rates for refresh.
Still further objects of the present invention are to provide apparatus and a method for transfer of information to a display which is capable of permitting refreshing of the display, which is capable of providing data transfer which is asynchronous with such display and its refresh, and which does not require a high data transfer rate to accomplish refreshing of the display.